G40.842 ICD-10-CM Code: KCNQ2-related epilepsy, not intractable, without status epilepticus
HCC Buddy Code Card
Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.
FY 2026 Apr update / Diseases of the nervous system (G00-G99) / Episodic and paroxysmal disorders (G40-G47)
G40.842
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceKCNQ2-related epilepsy, not intractable, without status epilepticus
Epilepsy caused by a KCNQ2 gene mutation that can be controlled with medication, not currently in a prolonged seizure state.
Buddy Insight
KCNQ2-related epilepsy, medication-responsive, without status epilepticus.
CMS-HCC V28
MappedHCC 201
RAF 0.262
CMS-HCC V24
MappedHCC 79
RAF 0.244
ACA/HHS
00
RAF 0
ESRD/PACE
MappedHCC 79
RAF 0.0
RXHCC
MappedHCC 164
RAF 0.0
Code Trumping
Basket needed
Code Book Path
Inclusion Terms
Official- KCNQ2-related epilepsy NOS
Excludes 2
OfficialICD-10-CM does not list Excludes 2 notes for G40.842 in this effective period.
Related Child Codes
Includes
OfficialICD-10-CM does not list Includes notes for G40.842 in this effective period.
Excludes 1
OfficialICD-10-CM does not list Excludes 1 notes for G40.842 in this effective period.
Code First
OfficialICD-10-CM does not list Code First sequencing instructions for G40.842 in this effective period.
Use Additional
OfficialICD-10-CM does not list Use Additional Code instructions for G40.842 in this effective period.
Code Also
OfficialICD-10-CM does not list Code Also instructions for G40.842 in this effective period.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.
Is G40.842 an HCC code?
Yes. G40.842 maps to Seizure Disorders and Convulsions under the CMS-HCC V28 risk adjustment model (and Seizure Disorders and Convulsions under V24).
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
MEAT Criteria for G40.842
For G40.842to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.
- MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
- EEvaluate: test results, medication response, or physical findings reviewed by the provider
- AAssess: explicit mention in the assessment or plan with acknowledgment of status
- TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed G40.842 during that encounter, not just copy-forwarded from a problem list.
What This Code Means
G40.842 is the ICD-10-CM diagnosis code for kcnq2-related epilepsy, not intractable, without status epilepticus. Epilepsy caused by a KCNQ2 gene mutation that can be controlled with medication, not currently in a prolonged seizure state. G40.842 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering episodic and paroxysmal disorders (g40-g47).
Under the CMS-HCC V28 risk adjustment model, G40.842 maps to Seizure Disorders and Convulsions (HCC 201) with a community, non-dual, aged base RAF weight of 0.262. Under the older V24 model, G40.842 mapped to the same category but with a base RAF weight of 0.244, V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
Document genetic confirmation of KCNQ2 mutation in the medical record. Because G40.842 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for G40.842 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Document genetic confirmation of KCNQ2 mutation in the medical record
- •Verify seizures are controlled with current medication regimen
Clinical Significance
KCNQ2-related epilepsy, medication-responsive, without status epilepticus. Many KCNQ2 patients with benign familial neonatal epilepsy have excellent seizure control and may outgrow seizures. However, some KCNQ2 variants cause KCNQ2 encephalopathy with persistent seizures and developmental issues. The specific mutation influences prognosis and treatment response.
Documentation Requirements
- ✓Genetic confirmation of KCNQ2 mutation
- ✓Seizure control status and medication response documentation
- ✓Absence of status epilepticus
- ✓Current antiepileptic medication regimen
- ✓Developmental milestones and neurological examination
- ✓Family history of neonatal seizures if relevant
Commonly Confused Codes
- •G40.841 — KCNQ2 epilepsy, not intractable, WITH status epilepticus
- •G40.844 — KCNQ2 epilepsy, INTRACTABLE, without status epilepticus
- •G40.802 — Other epilepsy, not intractable; less specific when KCNQ2 is confirmed
- •G40.909 — Unspecified epilepsy; avoid when genetic diagnosis is available
- •G40.42 — CDKL5 Deficiency; different gene mutation